<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    div {
      margin: 0 auto;
    }
    .box {
      width: 0;
      height: 0;
      /* background-color: skyblue; */
      border-top: 10px solid transparent;
      border-right: 10px solid transparent;
      border-bottom: 20px solid rgba(0, 0, 0, .6);
      border-left: 15px solid transparent;
    }

    .test {
      width: 100px;
      height: 50px;
      background-color: rgba(0, 0, 0, .6);
      border-radius: 10px;
      text-align: center;
      line-height: 50px;
      color: #fff;
    }
  </style>
</head>
<body>
  <div class="box"></div>
  <div class="test">消息框</div>
</body>
</html>